A cupola is either a refractory lined or a water cooled vertical shaft open at the top and bottom used for melting scrap metal such as iron and steel for subsequent use in castings. An outer metal plate shell provides protection and support for the materials charged to the cupola that are processed in the cupola. In operation, coke is charged to the cupola and extends upward a predetermined height from the bottom of the cupola to form a coke bed. Next, the coke bed is ignited and alternate charges of flux material, scrap metal and coke are charged to the cupola to a predetermined height. Forced air is then added to the burning coke bed through tuyeres or openings which extend through the metal plate shell and refractory or water cooled wall to the combustion zone just above the coke bed such that combustion occurs rapidly within the coke bed. The air, which may be preheated, is supplied from a suitable blower through a blast pipe to an equalizing chamber called a windbox. The windbox completely encircles the cupola to supply air uniformly to the tuyeres. The coke is burned with the forced air to produce high temperature products of combustion. The high temperature products of combustion inherently ascend through the scrap metal, exchange heat with the scrap metal and melt the scrap metal and then exit out the top of the cupola at a relatively lower temperature.
As the coke burns a sizable and constant thermal gradient is created between the portion of the metal scrap nearest the burning coke bed and the scrap metal nearest the top of the stack. The hottest scrap metal on the burning coke bed melts becoming more fluid and descends through the burning coke thereby dissolving the carbon from the coke. In this manner, as the scrap metal melts and descends to the bottom of the cupola the balance of the scrap metal on top of the coke bed also descends downward in the vertical shaft of its own weight such that a new portion of solid scrap metal automatically settles into the melting zone and room is continually made available for the addition of new scrap metal at the top of the cupola. The melted scrap metal which descends to the bottom of the cupola may be collected in a well at the base of the coke bed and periodically tapped as needed. The composition of the melted scrap metal collected in the well is determined by the composition of the charges and by the various reactions that take place during and after melting.
As previously described, limestone and other suitable fluxes are continually charged to the cupola along with the scrap metal to remove the ash containing oxides and other unwanted materials remaining from the combustion process and prevent the ash from accumulating and blocking the shaft and rendering the cupola inoperative. The ash is calcined upon arrival at the coke bed such that the ash may be removed from the cupola as slag. The calcium oxide reacts with the ash to produce heat and fluid calcium silicate which drips down through the bed of burning coke. Being lighter than the molten metal, it floats to the top of the well and is then removed.
Traditionally, cupolas have used coke as both a fuel and as a support for the charge of iron and limestone. Notwithstanding, the use of coke as a fuel is less than satisfactory because coke is continually increasing in cost due to the relative scarcity of good coking coals. In addition, coke naturally adds impurities such as sulfur to the product metal, and causes emissions of sulfur oxides and fine particles. Accordingly, there have been a number of attempts to eliminate coke for iron melting.
One attempt to eliminate coke for iron melting involves the use of gas as the only fuel. The use of gas is desirable because it is virtually sulfur free and also eliminates the majority of undesirable emissions, especially dust. However, there are a number of problems which affect product quality associated with the use of a gaseous fuel. For example, the use of gas as the only fuel results in a loss of carbon from the metal. Furthermore, in a gas fueled cupola it is difficult to achieve adequate metal temperature because heat transfer from a hot gas produced by a gas fuel is less effective than direct contact heating with coke in a coke fueled cupola.
In an attempt to overcome the deficiencies of a coke fueled or gaseous fueled cupola, it has been found that gaseous fuel may be used to replace at least 25 percent of the energy typically supplied in a coke fueled cupola with minimal effect on metal quality and overall thermal performance of the cupola. Accordingly, by replacing part of the coke fuel with gas, the total impurity and emission levels may be reduced, quality of the product metal improved and manufacturing costs decreased, e.g., gas is currently available at about 60% of the cost of coke. Supplemental gas fuel firing of the cupola also results in increased productivity and throughput of the cupola. A gas supplemented cupola allows the upper preheating zone of the cupola to be optimized independently at a higher blast rate when compared with the lower reduction and melting zones of the cupola. For example, typical foundries that use a significant amount of steel in the making of iron operate with a blast rate of 325 cfm/ft.sup.2 in the lower zones. The use of gas supplemented cupolas allows the blast rate to be increased to more than 325 cfm/ft.sup.2 in the preheating zone.
Although, it has been found that improved efficiency and productivity may result from the use of a supplemental fuel such as gas in a coke fueled cupola, precision control of gas combustion is required for efficient cupola operation. The water formed by gas combustion has been found to react endothermically with the coke in the cupola thereby reducing the reaction temperature in the cupola. Furthermore, the available oxygen in the cupola for combustion has been found to react with the coke in preference to the gas thereby rendering the addition of gas largely ineffective.
In addition, standard burner control technology typically controls the air/fuel ratio ("A/F ratio") to the burner by a pressure signal from the air flow conduit to a fuel flow regulator. The pressure balance system operates by setting an air flow valve within the air flow conduit in a predetermined set position required for operation. Downstream of the air flow valve is a pressure line in communication with a fuel flow valve to regulate the flow of fuel within a fuel conduit to the burner as a function of the air flow pressure in the air flow conduit. As the air flow is increased or decreased the pressure in the air flow conduit is increased or decreased thereby increasing or decreasing the flow of fuel.
It will be appreciated that the pressure balanced system is susceptible to changes in the air/fuel ratio when obstructions in the burner nozzle produce back pressure in the combustion air conduit and cause an increased rate of fuel flow while the air flow is actually reduced. Accordingly, the burner flame on heretofore known cupolas quite often "blows out" or operate "off-ratio" when raw material charges are dropped into the cupola in and around the burners. More particularly, the manner that the raw material charges are frequently added to the cupola results in a non-uniform charge depth or permeability of the raw material above the burners in the cupola. The nonuniform depth of raw material or permeability of charge results in back pressure variations above the burners. The burners, adversely affected by variations of back pressure created by the non-uniform arrangement of the raw material above the burners causes the burner to "blow out" or operate inefficiently because of an incorrect air/fuel ratio resulting in reduced flame temperature and stability. Unless indicated otherwise, as used herein, the term "fuel" refers to gaseous or liquid fuels such as natural gas, fuel oil and the like.
The problem of variable back pressure is compounded because the standard burner air/fuel control strategy is to manifold the burners so as to have one balanced pressure control for two or more burners of the cupola. As the back pressure varies from burner to burner, the air supplied to the burners tends to vary with the most amount of air flowing to the burner with the least back pressure resistance. Accordingly, the air flow and fuel flow and air/fuel ratio to the multiple burners is not acceptable because the burners bias on their own.
In addition to the problem of variable back pressure on the operation of the burner control system caused by the cupola charge, attachment of the burners to the cupola at a 90.degree. angle to the cupola side wall also facilitates the build-up of molten and solid material in the burner tunnel. As previously described, build-up of molten and solid material in the burner tunnel adversely affects the air/fuel ratio thereby contributing to burner failure. In those instances where the build-up of material does not directly cause the burner to fail, the build-up of material does indirectly cause back pressure variations in the cupola thereby generating an inappropriate pressure signal to the burner causing it to fail or function improperly. Thus, this two fold problem requires frequent expensive maintenance down time to clean out the burner tunnels to maintain proper burner operation.
To alleviate the aforementioned problems we have invented an apparatus and a method for controlling the operation of each burner for a cupola. The system addresses flow pressure across the tunnel of each burner as well as air/fuel ratio pressure at the burner such that each burner is operatively controlled for constant burning under a variety of adverse operating conditions.